32 



THE GLACIAL EPOCH. 



Post-Glaoial 

 modifica- 

 tion. 



Glacial 

 theory of 

 dispersal. 



been discovered by this species, or that the birds wintering in these three isolated localities 

 meet in summer on a common breeding-ground. 



Modification is probably proceeding very slowly with these species at the present time ; 

 they are all migratory birds ; at their breeding-grounds they have an unlimited supply of 

 food and very few enemies ; their great struggle for existence is the necessity for braving 

 the perils of migration twice a year, and the survivors are those who are fittest to run the 

 gauntlet of storms. But there is evidence of considerable post-glacial modification. 



The Curlew {Numenius arquatus), which winters in Africa, is slightly different from its 

 oriental representative {Numenius arquatus lineatus), which winters in Australia, and the 

 European Dunlin {Tringa alpina) is said to differ from its representative in America. It is 

 impossible to say to what extent post-glacial modification may have taken place, because in 

 the absence of isolation it has not produced differentiation. In what manner and to what 

 extent isolation has permitted (not caused) differentiation to take place in the family of 

 Charadriidse is the object of the present book. How far I have succeeded in the task I 

 must leave my readers to decide. I hope they will agree with me that the circumstantial 

 evidence which I have collected on tlie subject is sufficient to warrant my giving it the title 

 of the Glacial Theory of Dispersal. At all events I trust they will neither hastily condemn 

 my ideas as " unverified hypotheses of speculative philosophy," nor rush into the opposite 

 extreme, and jump to the conclusion that the Glacial Epochs have been an important factor 

 in the differentiation of tropical and subtropical genera and species. 



